1. Field of the Invention
The present invention is directed to methods for testing light-emitting displays on-line during a continuous fabrication process.
2. Description of Related Art
In a typical plasma display, a gas or mixture of gases is enclosed between orthogonally crossed and spaced conductors. The crossed conductors define a matrix of crossover points, arranged as an array of miniature picture elements (pixels), which provide light. At any given pixel, the orthogonally crossed and spaced conductors function as opposed plates of a capacitor, with the enclosed gas serving as a dielectric. When a sufficiently large voltage is applied, the gas at the pixel breaks down creating free electrons that are drawn to the positive conductor and positively charged gas ions that are drawn to the negatively charged conductor. These free electrons and positively charged gas ions collide with other gas atoms causing an avalanche effect creating still more free electrons and positively charged ions, thereby creating plasma. The voltage level at which this ionization occurs is called the write voltage.
Upon application of a write voltage, the gas at the pixel ionizes and emits light only briefly as free charges formed by the ionization migrate to the insulating dielectric walls of the cell where these charges produce an opposing voltage to the applied voltage and thereby extinguish the ionization. Once a pixel has been written, a continuous sequence of light emissions can be produced by an alternating sustain voltage. The amplitude of the sustain waveform can be less than the amplitude of the write voltage, because the wall charges that remain from the preceding write or sustain operation produce a voltage that adds to the voltage of the succeeding sustain waveform applied in the reverse polarity to produce the ionizing voltage. Mathematically, the idea can be set out as Vs=Vw−Vwall, where Vs is the sustain voltage, Vw is the write voltage, and Vwall is the wall voltage. Accordingly, a previously unwritten (or erased) pixel cannot be ionized by the sustain waveform alone. An erase operation can be thought of as a write operation that proceeds only far enough to allow the previously charged cell walls to discharge; it is similar to the write operation except for timing and amplitude.
Typically, there are two different arrangements of conductors that are used to perform the write, erase, and sustain operations. The one common element throughout the arrangements is that the sustain and the address electrodes are spaced apart with the plasma-forming gas in between. Thus, at least one of the address or sustain electrodes is located within the path the radiation travels, when the plasma-forming gas ionizes, as it exits the plasma display. Consequently, transparent or semi-transparent conductive materials must be used, such as indium tin oxide (ITO), so that the electrodes do not interfere with the displayed image from the plasma display. Using ITO, however, has several disadvantages; for example, ITO is expensive and adds significant cost to the manufacturing process and ultimately the final plasma display.
The first arrangement uses two orthogonally-crossed conductors, one addressing conductor and one sustaining conductor. In a gas panel of this type, the sustain waveform is applied across all the addressing conductors and sustain conductors so that the gas panel maintains a previously written pattern of light-emitting pixels. For a conventional write operation, a suitable write voltage pulse is added to the sustain voltage waveform so that the combination of the write pulse and the sustain pulse produces ionization. In order to write an individual pixel independently, each of the addressing and sustain conductors has an individual selection circuit. Thus, applying a sustain waveform across all the addressing and sustain conductors, and applying a write pulse across only one addressing and one sustain conductor will produce a write operation in only the one pixel at the intersection of the selected addressing and sustain conductors.
The second arrangement uses three conductors. In panels of this type, called coplanar sustaining panels, each pixel is formed at the intersection of three conductors, one addressing conductor and two parallel sustaining conductors. In this arrangement, the addressing conductor orthogonally crosses the two parallel sustaining conductors. With this type of panel, the sustain function is performed between the two parallel sustaining conductors and the addressing is done by the generation of discharges between the addressing conductor and one of the two parallel sustaining conductors.
The sustaining conductors are of two types, addressing-sustaining conductors and solely sustaining conductors. The function of the addressing-sustaining conductors is twofold: to achieve a sustaining discharge in cooperation with the solely sustaining conductors; and to fulfill an addressing role. Consequently, the addressing-sustaining conductors are individually selectable so that an addressing waveform may be applied to any one or more addressing-sustaining conductors. The solely sustaining conductors, on the other hand, are typically connected in such a way that a sustaining waveform can be simultaneously applied to all of the solely sustaining conductors so that they can be carried to the same potential in the same instant.
Numerous types of plasma panel display devices have been constructed with a variety of methods for enclosing a plasma-forming gas between sets of electrodes. In one type of plasma display panel, parallel plates of glass with wire electrodes on the surfaces thereof are spaced uniformly apart and sealed together at the outer edges with the plasma-forming gas filling the cavity formed between the parallel plates. Although widely used, this type of open display structure has various disadvantages. The sealing of the outer edges of the parallel plates and the introduction of the plasma-forming gas are both expensive and time-consuming processes, resulting in a costly end product. In addition, it is particularly difficult to achieve a good seal at the sites where the electrodes are fed through the ends of the parallel plates. This can result in gas leakage and a shortened product lifecycle. Another disadvantage is that individual pixels are not segregated within the parallel plates. As a result, gas ionization activity in a selected pixel during a write operation may spill over to adjacent pixels, thereby raising the undesirable prospect of possibly igniting adjacent pixels. Even if adjacent pixels are not ignited, the ionization activity can change the turn-on and turn-off characteristics of the nearby pixels.
In another type of known plasma display, individual pixels are mechanically isolated either by forming trenches in one of the parallel plates or by adding a perforated insulating layer sandwiched between the parallel plates. These mechanically isolated pixels, however, are not completely enclosed or isolated from one another because there is a need for the free passage of the plasma-forming gas between the pixels to assure uniform gas pressure throughout the panel. While this type of display structure decreases spillover, spillover is still possible because the pixels are not in total electrical isolation from one another. In addition, in this type of display panel it is difficult to properly align the electrodes and the gas chambers, which may cause pixels to misfire. As with the open display structure, it is also difficult to get a good seal at the plate edges. Furthermore, it is expensive and time consuming to introduce the plasma-forming gas and seal the outer edges of the parallel plates.
In yet another type of known plasma display, individual pixels are also mechanically isolated between parallel plates. In this type of display, the plasma-forming gas is contained in transparent spheres formed of a closed transparent shell. Various methods have been used to contain the gas-filled spheres between the parallel plates. In one method, spheres of varying sizes are tightly bunched and randomly distributed throughout a single layer, and sandwiched between the parallel plates. In a second method, spheres are embedded in a sheet of transparent dielectric material and that material is then sandwiched between the parallel plates. In a third method, a perforated sheet of electrically nonconductive material is sandwiched between the parallel plates with the gas-filled spheres distributed in the perforations.
While each of the types of displays discussed above are based on different design concepts, the manufacturing approach used in their fabrication is generally the same. Conventionally, a batch fabrication process is used to manufacture these types of plasma panels. As is well known in the art, in a batch process individual component parts are fabricated separately, often in different facilities and by different manufacturers, and then brought together for final assembly where individual plasma panels are created one at a time. Batch processing has numerous shortcomings, such as, for example, the length of time necessary to produce a finished product. Long cycle times increase product cost and are undesirable for numerous additional reasons known in the art. For example, a sizeable quantity of substandard, defective, or useless fully or partially completed plasma panels may be produced during the period between detection of a defect or failure in one of the components and an effective correction of the defect or failure.
This is especially true of the first two types of displays discussed above; the first having no mechanical isolation of individual pixels, and the second with individual pixels mechanically isolated either by trenches formed in one parallel plate or by a perforated insulating layer sandwiched between two parallel plates. Due to the fact that plasma-forming gas is not isolated at the individual pixel/subpixel level, the fabrication process precludes the majority of individual component parts from being tested until the final display is assembled. Consequently, the display can only be tested after the two parallel plates are sealed together and the plasma-forming gas is filled inside the cavity between the two plates. If post-production testing shows that any number of potential problems have occurred (e.g. poor luminescence or no luminescence at specific pixels/subpixels), the entire display is discarded.